Process for removing nitric oxide from gaseous mixtures



3,428,414 PROCESS FOR REMOVING NITRIC OXIDE FROM GASEOUS MIXTURES Werner H. Baum, Monroeville Borough, Allegheny County, Joseph G. Crist, Mount Lebanon Township, Allegheny County, and Elliott V. Nagle, Franklin Township, Westmoreland County, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed June 2, 1966, Ser. No. 554,685 US. C]. 23-22 9 Claims Int. Cl. C01b 21/40, 21/46 The present invention relates to the removal of nitric oxide (NO) from gaseous mixtures. The invention is concerned with the removal of NO from gaseous mixtures containing oxygen and reactive organic material, more particularly, coke-oven gases or the like.

The NO present as a contaiminant in coke-oven gas or the like is known to react with oxygen and other organic material to form complex materials, usually called gums. Gums cause clogging of equipment and pipe lines. Gums that form in apparatus for the cryogenic separtion of gaseous mixtures are known to cause explosions.

It is known to convert or remove NO in these gases by the oxidation thereof to nitrogen dioxide (N Any N0 that is formed ordinarily reacts immediately with reactive organic material to form organic-nitrogen-oxygen compounds. This gummy, complex nitrogeneous organic material tends to precipitate from the gas and may be removed together with any remaining N0 by known methods, :for example, by wet or dry scrubbing. Hence, the troubles may be avoided that occur when gum is allowed to form and accumulate where it will. This may be done by causing gum to form where it is innocuous and may be conveniently removed from the system.

In general, such a process for treating a ga mixture to oxidize NO to N0 and form organic-nitrogen-oxygen compounds comprises retaining the gas mixture for a short time in a reaction chamber at an elevated temperature, for example, at a temperature between about 60 and 90 C. The reactions involved and the formed organicnitrogen-oxygen compounds are complex. Perhaps for these reasons the process has been found to be sluggish or non-operative with some gas mixtures, The process may even cease altogether for no apparent reasons. A number of suggestions have been made to render the process more reliable. For example, it has been proposed to subject the gas to a brush discharge or to irradiation with a high-pressure mercury-vapor lamp.

We have now found another way to make this process more reliable with respect to initiating and sustaining the reaction or reactions to insure NO removal or conversion. According to our invention, a small quantity of an N0 containing gas is admixed with the particular gas containing NO, oxygen and reactive organic material. The admixture is then flowed through a heated reaction chamber and is allowed to reside therein fora short time at a temperature between about 50 and 110 C. Alternatively, N0 may be injected into the chamber to admix with the gas slowly flowing therethrough. The chamber need not be heated, however, where the inlet gas temperature maintains the admixture in the chamber between about 50 and 110 C.

We have found by adding N0 continuously to the flowing gas, the reactions that eliminate NO are catalyzed or initiated rather quickly and are sustained. Since it is known that N0 reacts quite rapidly with the constituents in coke-oven gas, for example, those skilled in the art could reason that any N0 added to the gas should disappear rapidly and have no effect on the rate of NO conversion. Surprisingly, we have found this is not the case.

United States Patent 0 ICC Our process may be operated within the following limits or ranges. As indicated above, the temperature in the reaction chamber may vary between about 50 and 110 C., the preferred range being between about 60 and C. Below 50 C. and above C. the conversion of NO to N0 gradually decreases. We have found that at C. the conversion rate is very low.

The concentration of N0 for substantially complete (90%) elimination of NO may vary between about 40% and 80% of the concentration of NO. That is, the volume ratio of N0 to NO may vary between about .40 and .80 to rapidly initiate and sustain NO elimination at the optimum conditions of temperature and residence time. As the ratio approaches .40 and decreases to about .30, the residence time should be on the high side of its range. Increasing the residence time therebeyond is satisfactory for treating small volumes, but becomes more uneconomical as the gas volume increases. Likewise, below a ratio of .30 to about .06, the reaction chamber may require conditioning or preparation for sustained, optimum NO elimination. While we do not know the mechanism involved, conditioning may be provided by passing the gas through the elevated-temperature reaction chamber for some time, which may be more than an hour. Above a ratio of .80 to about 1.00, NO elimination improves somewhat. Beyond a raio of 0.80 and the added N0 cost should be balanced against higher N0 conversion.

An NO -containing gas useful in our process may be N0 itself or a gas such as nitrogen tetroxide, a mixture comprising N0 in equilibrium with N 0 In any event, sufiicient gas should be used to provide the required concentration of N0 In the known oxidation reaction of NO to N0 it has been stated that at least 01% oxygen by volume should be present. We have operated our process satisfactorily in a range between about 0.19% and 5% oxygen by volume. For example, at substantially the same optimum temperature and residence time and at an NO /NO ratio between about 0.34 and 0.38, the NO conversion was 76% with 0.19% oxygen and 97% with 1.1% oxygen by volume.

A great number of materials are known to react with N0 or interact with NO and oxygen, under the conditions of temperature and pressure suitable for an NO conversion process such as ours. It is to be understood that the term reactive organic material as used throughout this specification and the claims includes effective materials, such as dienes, conjugated dienes, unsaturated hydrocarbons and hydrocarbons, that react with oxygen, NO and N0 to form complex org-anic-nitrogemoxygen compounds. For example, just one paper discusses more than thirty compounds in the above categories found to react with N0 to form gum [C. W. Jordan, A. L. Ward and W. H. Fulweiler, Gum Deposits in Gas Distribution Systems, Vapor Phase Gum, Industrial and Engineering Chemistry 26, 947 (1934)]. Of the known compounds, all we have tested have been found effective, including 1,3-butadiene, cyclohexene, cyclopentadiene and m-xylene. Our process requires a minimum concentration by volume of reactive organic material. For example, we have operated our process satisfactorily with parts per million (p.p.m.) by volume of 1,3- butadiene. Other conditions remaining constant, we have found that an increase in the 1,3-butadiene concentration from 150 ppm. to 200 ppm. increased'NO conversion from about 80% to about 90%. The reactive organic material found in coke-oven gas or the like is usually in excess of minimum requirements but may, of course, be increased in accordance with our teachings.

Our process operates successfully when an NO -enriched gas resides for a short time in an elevated temperature reaction chamber. We have found a time be- 3 4 tween about 8 seconds and 16 seconds to be satisfactory. TABLE I Below about 8 seconds the NO elimination decreases Gas outofTube quite rapidly. When a gas takes longer than 16 seconds Example Temperature, NO to traverse a heated chamber the NO elimination is 1mo, c i z proved, as discussed above with respect to the NO /NO Percen ratio. The capital cost increases, however, since a larger g: 3 2g 3% chamber must be provided. 88 0: 4 97 Our process is applicable to gases containing NC, for 100 3.8 68 example, those resulting from high temperature reactions where air has been present. The carbonization of It m y be $6611 from the above x p a f y coal and resulting coke-oven gas is but one example. be converted removed r a g sontalpmg by Other gases include crude synthesis gases obtained by admlxtllre thel'fiwlth 2 an Noz/No ratio partial oxidation of natural gas, crude oil, coal, refinery 111 the absence of 2, y 11316 No was convel'tedgas, naphtha or gas oil. A gas may contain NO, dglde to E l 5-12 the raw materials used or due to its processing. any

Another gas representatwe of a hydrogen-rlch fractlon of these gases are sub ected to low-temperature operaf coke-oven gas was treated at 155 pounds per square trons or cr 0 enic se aratlon re uirm nor removal of 9 9 NO Natur ali y or d iie to proc z essing grior to NO remch.gage m an apparatus cqmpnsmg a i g moval, these gases may not have the preferred conceni g' 5 $g g s g gg g g i trations of oxygen and reactive organic material. Sur- C a C a f prisin ly some N0 may be present It is evident from having an Inside dlameter of 1/32 Inch i a capaclltyg D 7 n the above description, however, that any deficiency in 100.1111 f P q chamber a Stainless Stee N02 oxygen or reactive organic material may be cop havmg an 1ns1de diameter of 1.8 inches and a capatcity rected prior to NO conversion according to our process. of It was pagked i ii E so at We prefer to construct the reaction chamber of carthe volddsllace g t iii? bon steel rather than stainless steel. Under the varying conPecte y Cu W0 e6 0 4 me 5 a e S conditions described above, we have found that with tubing Wham} the gas usually cooled to about g carbon steel as opposed to stainless steel, the reaction temperature Slmultaneous gas Samples could be taken is initiated more rapidly Any required chamber com before and after the heated chamber and after thepac ed ditioning is more rapid. Where operating conditions chamber The gas cofnPnsed by volume 69% mtmgen change, the reaction stabilizes itself more rapidly. It 2 i fg gi 'j fi i gg iggj should be pointed out, however, that with an extended O0 u a lens an e a f 0g chamber conditioning period but with otherwise opti tameters were used to control the residence tune and the mum conditions, an NO elimination of about 86% was content f P i i g f i obtained with a stainless steel chamber at an NO /NO heated to m t ea e 0 am an 00 a 9 ratio of about 006 Hence, Where a gas Corrosive to 10.7 seconds to traverse the chamber before traversmg carbon steel must be treated, a stainless steel chamber the mbmg and Packet? chamber The gas flowmg to the may be more Satisfactory heated chamber contamed NO and NO. After an elapsed A complete understanding of the invention may be time of one 1 1; at dapplrloxugtately constgnto rggndltions obtained from th follow' t ical exam les. 40 the gas to t e eats c am er contame I 6 mg yp P N0 and 1.12 p.p.m. NC for an NO /NO ratio of 0.79.

EXampleS A sample taken at the same time, after the packed cham- A gas representative of a hydrogen-rich, tail gas fracber, ShQWed P-P- NO an NO Converslon of tion of coke-oven gas, from which hydrogen is recovered Simultaneous Samples thereafter taken t by a li f ti Process, comprised by volume 8.7 and 9.2 hours, the elapsed tlmes after the beginning (12 p.p.m.) NO, 5.0% oxygen, 0.24% 1,3-butadiene, 0f the test- 28.7% methane, 2.7% nitrogen, 6.4% carbon monoxide The general procedure described in Example 5 was reand the remainder hydrogen. Admixed therewith was peated for Examples 7-12, but with varying operating 0. 00044% (4.4 p.p.m.) N0 representing an NO /NO conditions. In Example 6 the injection of N0 was stopped ratio of 0.37. At atmospheric pressure, the admixture and the NO content of the gas was raised to 2.14 p.p.m. was passed through a heated reaction chamber compris- The NO removal or conversion stopped. After four hours ing a Pyrex glass tube having a volume of 117 ml. Heated at these conditions the gas after the packed chamber to about 44 C., the residence time of the admixture was contained 2.14 p.p.m. NO, there being no conversion 15 seconds. The admixture out of the tube analyzed 3.8 thereof. The operating conditions in each example were p.p.m. NC for an NO conversion of 68%. Example '1 maintained as constant as possible. The operating conwas repeated as Examples 2-4, only the temperature ditions and test results obtained are summarized in being changed. The results are summarized in Table I. Table II.

TABLE II T P D Heaten Reaction Chamber Elapsed est enod am Example m CTirdne at Gas Into Heated Chamber After Packed Chamber 651 61109 On 1 10118, C. time, seconds hours N 02 NO, N 0g/ NO N 0, N 0 Conversion p.p.m p.p.m Ratio p.p.m percent 1 0.89 1.12 0. 79 0.08 93 10 7 5.5 0. 64 1.15 0,56 0.11 91 3. 7 0. 64 1.13 0. 57 0.12 89 75 1o 7 2' 8 62 8 0. 07 0170 0193 0:75 0134 53 75 12 7 3.33 0.70 0.89 0.79 0.0 100 88 8'38 88% 8'88 882 88 75 14 1' 01 4s 1: 17 0: 39 0110 91 12 8'? 8 28 1'88 88? 8'88 88 50 9. 2 1' 0: 04 11 55 0141 11 1s 24 50 14 2. 25 0. 77 1. 34 0. 58 0. 50 63 60 14 1. 5 0. 77 1. 31 0. 59 0. 20

* N 0 test results from sample taken after heated reaction chamber.

It may be seen from the above examples that Example 5 illustrates some conditions we have found to be optimum in our preferred operating ranges for 90% conversion of NO. Example 6 shows that in the absence of N0 it was not possible to convert NO. Example 7 illustrates 100% NO conversion by providing a greater NO2/ NO ratio and a somewhat longer residence time than in Example 5. After 6.1 hours, simultaneous samples were taken before and after the heated chamber. Conversion in the heated chamber was 82% with further conversion taking place when the gas was thereafter retained for a short time at a temperature below its temperature in the heated chamber. The packed chamber simulated any processing equipment that may be used for reaction completion and/ or to remove reaction products from a gas, for example, a liquid or dry scrubber or a chamber which may also serve as a surge tank ahead of a liquefaction step. Depending on such factors as the extent of NO conversion required, changes in gas composition or changes in operating conditions, it is possible to achieve the benefits of our process by using only a heated chamber or by supplementing this chamber with known means that provide time for reaction completion as well as removal of reaction products.

Examples 8-12 illustrate operation at the low side of the preferred range of NO /NO ratios, and the effect of changes in temperature and residence time. Example 8 shows that with an NO /NO ratio of 0.39, an NO conversion of 91% was obtained when the residence time was increased to the high side of this preferred range at 14 seconds. Example 9 illustrates that at 110 C., the NO conversion dropped to 50% at an NO N 0 ratio of 0.36, but increased to 61% when the NO /NO ratio was increased to 0.41. Example 10 illustrates that a 50 C. and a residence time of 9.2 seconds the NO conversion dropped to 24% at an NO /NO ratio of 0.41. When the Example 10 conditions were changed by increasing the residence time and NO /NO ratio, the conversion increased to 63% as illustrated by Example 11. When the Example 11 conditions were changed by increasing the temperture to 60 C., the NO conversion increased to 85% as illustrated by Example 12.

While the above examples illustrate preferred methods and conditions of operation within our broad operating limits, it is obvious therefrom that changes may be made therein without departing from the spirit of the invention. It will be apparent that, for commercial operation of our process, the hereinabove described equipment may be of any design known to eflect the desired results.

Although we have disclosed herein the practice of our invention, we intend to cover as well any changes or modifications therein which may be made without departing from the spirit and scope of the invention.

We claim:

1. In a process for the removal of nitric oxide from a gas mixture containing oxygen and reactive organic material, including retention of the gas mixture for a short time in a reaction chamber at an elevated temperature, the improvement comprising retaining said gas mixture in said chamber in admixture with a gas containing nitrogen dioxide, the volumetric ratio of nitrogen dioxide to nitric oxide being at least about 0.06.

2. The process of claim 1 wherein said ratio is at least about 0.30.-

3. The process of claim 1 wherein said ratio is between about 0.30 and 1.00.

4. The process of claim 1 wherein. said admixture is retained in said chamber at a temperature between about and C.

5. The process of claim 1 wherein. said admixture is retained in said chamber for at least about 8 seconds.

6. The process of claim 1 wherein said admixture comprises at least about 0.1 percent oxygen by volume.

7. The process of claim 1 wherein said admixture comprises by volume at least about parts per million of a reactive organic material.

8. The process of claim 1 wherein. said admixture is thereafter retained for a short time at a temperature below the temperature of said admixture in said chamber.

9. The process of claim 1 wherein said gas mixture comprises a hydrogen-rich fraction of coke-oven gas.

US. Cl. X.R. 23-159 

1. IN A PROCESS FOR THE REMOVAL OF NITRIC OXIDE FROM A GAS MIXTURE CONTAINING OXYGEN AND REACTIVE ORGANIC MATERIAL, INCLUDING RETENTION OF THE GAS MIXTURE FOR A SHORT TIME IN A REACTION CHAMBER AT AN ELEVATED TEMPERATURE, THE IMPROVEMENT COMPRISING RETAINING SAID GAS MIXTURE IN SAID CHAMBER IN ADMIXTURE WITH A GAS CONTAINING NITROGEN DIOXIDE, THE VOLUMETRIC RATIO OF NITROGEN DIOXIDE TO NITRIC OXIDE BEING AT LEAST ABOUT 0.06. 